Stroke has nothing to do with torque.
Here are some data.
090 has 8.6 HP with 9.5 Nm (7 ft-lb) of torque at 137cc. Bore 66mm Stroke 40mm
070 has 6.5 HP with 7.7 Nm (5.7 ft-lb) of torque at 106cc. Bore 58mm Stroke 40mm
075/076 has 7 HP with 6.8Nm (5 ft-lb) of torque at 111cc. Bore 58mm Stroke...